The United States’ largest avian flu outbreak in decades is decimating poultry flocks in the heartland and shaking up old certainties about how highly pathogenic avian flu viruses spread. “All the old dogma about high-path influenza transmission has just gone out the window,” says Michael Osterholm, director of the Center for Infectious Disease Research and Policy here at the University of Minnesota (UMN), Twin Cities. “We’re in totally uncharted territory.”
More than 30 million poultry, mostly chickens and turkeys, have been affected, either infected directly by the lethal H5N2 virus or marked to be sacrificed in massive culls. Three states—Iowa, Minnesota, and Wisconsin—have declared a state of emergency, and the federal government has earmarked at least $330 million in emergency funds. So far, the virus appears to pose no threat to people, unlike some other avian flus. But its transmission route is a mystery, and infection control measures have failed to contain it. “This is an unprecedented outbreak in which influenza doesn’t seem to follow the rulebook,” says Jeff Bender, an epidemiologist at UMN Twin Cities.
The current outbreak, which has affected 143 backyard and commercial flocks since January, is the third arrival of high-pathogenicity H5N2 to the United States: Different strains walloped Virginia and Pennsylvania in 1983, affecting 17 million poultry, and a single flock of 6600 chickens in Texas in 2004. The latest one evolved from an H5N8 virus found in a migratory bird in Russia in September 2014. The same H5N8 strain later cropped up in Germany, Japan, the Netherlands, and the western United States (Science, 6 February, p. 616). Along the way, it swapped genes with a North American avian influenza virus to create the new H5N2 strain, says David Swayne, director of the U.S. Department of Agriculture’s Southeast Poultry Research Laboratory in Athens, Georgia.
First confirmed in poultry on U.S. soil on 3 January in samples collected from Washington state, the virus began infecting birds across the Pacific Northwest, mainly in backyard flocks. By early March, it had infected commercial turkeys in Minnesota’s Pope County, presumably carried there by wild birds. From there, it cropped up across the Midwest, hitting 12 states in total. Avian flu outbreaks normally slow as temperatures warm, but this time spring brought no reprieve. Nor do investigators understand why the virus is ravaging turkey farms, threatening so many birds that some industry observers have warned of a turkey shortage this Thanksgiving. Bird flu typically infects chickens more readily, Bender says. But the transmission route is the most troubling mystery.
Some scientists initially guessed that the virus was entering farms on wild bird feces stuck to equipment or workers’ clothing. “We were fairly clear that we were seeing point introductions,” says Beth Thompson, assistant director of the Minnesota Board of Animal Health in St. Paul. “These sites weren’t connected in any way.” Lately, the virus has spread more rapidly through nearby turkey farms, suggesting that it is windborne. “It could be a plume-related effect,” Osterholm says, in which a gust carries feathers or dust long distances. Other hypotheses for the virus’s spread include lapses in biosecurity and rodents tracking it onto farms.
Animal health officials have implemented stricter biosecurity measures like having workers wash boots and change clothes before entering a barn and mapped 10-kilometer-radius response zones around affected farms for surveillance and testing. Here in Minnesota, the state with the largest number of infected sites, scientists are searching for the virus in wild bird feces, dead birds reported by the public, and wild turkey carcasses felled by hunters.
The Animal and Plant Health Inspection Service, UMN, and the turkey industry are also joining forces on a massive case control study to understand how H5N2 is spreading. Researchers will soon fan out to at least 30 affected turkey farms and an equal number of farms that haven’t been hit by the outbreak. They will take stock of farm management practices, the type of feed and equipment used, the presence of wild birds, and the farms’ proximity to roads and waterways, among other factors.
The Southeast Poultry Research Laboratory is now testing a candidate poultry vaccine. The U.S. Centers for Disease Control and Prevention (CDC) in Atlanta is also developing candidate vaccine viruses for use in humans in case the disease spreads to people, as some avian flus have in the past. But CDC is “cautiously optimistic that we will not see any human cases” because the virus does not currently have genetic markers associated with increased severity in people, CDC’s Alicia Fry said at a 22 April press conference.
Another unknown is whether the virus will stick around in wild birds after the outbreak burns out, posing a continuing threat to flocks. “Maintenance of a high-path avian flu virus within wild birds is unprecedented,” says David Stallknecht, an epidemiologist at the University of Georgia, Athens. But H5N2 already has a track record of confounding expectations. “We have to keep an open mind,” Osterholm says. “The greatest enemy we have right now is dogma.”
Correction (22 May 2015): The H5N2 avian influenza virus was erroneously described as first detected on U.S. soil on 19 December in a backyard poultry flock in Oregon, and the total number of states the virus has hit was listed as 14. In fact, it was first confirmed in poultry on U.S. soil on 3 January in samples collected from Washington state, and the correct number of states it has affected is 12. A novel H5N8 virus to which H5N2 traces its lineage was first discovered on 19 December in Oregon, and the H5N8 and H5N2 viruses have together hit 14 states.